You can make a cartesian diver out of different things - a ketchup sachet, a pen top or a plastic pipette. First make sure your diver floats. Do this by placing it in a wide-necked container of water. It is ideal if you can get the diver to float below the water surface. This can be achieved by adding a paperclip to the ketchup sachet, or a blob of clay to the pen top.
If you are using a plastic pipette, cut off most of the long narrow end, leaving you with the wider part of the pipette. Now fill the pipette one-quarter full of water. You can make it float by changing the amount of water in the pipette.
Once you are sure the diver will float, carefully put it into the bottle of water. Top the bottle up until it is completely full and then put the lid on. If there are bubbles in the water, tap the bottle until they have gathered at the top, and then top up with water.
Press the sides of the bottle. Your diver should dive to the bottom of the bottle, and by slowly decreasing and increasing the pressure on the outside of the bottle you should be able to make the diver move up and down.
When you increase the pressure on the bottle, the water and the air are compressed. The air bubble in the ketchup sachet is forced into a smaller volume, making the sachet denser than the surrounding water. The sachet sinks. When the pressure is removed, the air expands and the diver returns to the top. In the case of the pen top, water is forced into the pen cap, compressing the gas inside. Once again, this makes the pen cap denser than the water around it and it sinks.
This experiment, and many others, can be found in The Little Book of Experiments. This free resource is being sent to primary schools as part of Planet Science.
Sophie Duncan is programme manager for science at the BBC www.bbc.co.ukscience